Amides having from 1 to about 10 carbon atoms are useful in preparing pharmaceuticals, agricultural chemicals and various polymers. For example poly(vinylamine) is a valuable polymer which is best prepared by hydrolysis of the polymer formed on polymerization of N-vinylformamide which in turn is prepared form a number of precursors, all of which are synthesized from formamide. The purity of any of the amide products presents a problem in downstream reactions and this is especially true in the case of N-vinylformamide which contains residual quantities of formamide. Separation of formamide from N-vinylformamide or from any of its precursors, such as N-(1-ethoxyethyl)formamide, is complicated by the polar nature of the compounds, their high boiling points which typically exceed 200.degree. C., and the thermal instability of the vinyl product and its precursors. In general, the art lacks suitable commercial methods for low temperature, non-distillation separation methods for purifying amides.
U.S. Pat. No. 4,334,097, Schmidt (1982), discloses preparation of N-(.alpha.-alkoxyalkyl)carboxamides which are useful intermediates in making N-vinyl-carboxamides, e.g., N-vinyl-N-methylacetamide, which can be polymerized to give valuable polymers. The N-(.alpha.-alkoxyalkyl)carboxamide is made by reacting a carboxylic acid amide with .alpha.-halogenoalkyl ethers in the presence of tertiary amines. Separations are performed by solvent extractions and distillations.
Japanese Patent Application No. 60-129132, Sato et al. (1985), discloses the separation of low levels of formamide from N-vinylformamide using solvent extraction. This process requires large quantities of aromatic hydrocarbon and the disposal of aqueous formamide waste streams.
Japanese Patent Application No. 61-28969, Sato et al. (1986), describes separating formamide from N-vinylformamide by extraction wit water and an aromatic hydrocarbon. The N-vinylformamide is recovered from the organic phase.
U.S. Pat. No. 4,567,300, Murao et al. (1986), describes making N-substituted formamides from formamide and acetaldehyde using a basic catalyst and, optionally, further reacting an alcohol in the presence of an acid catalyst to form N-(1-hydroxyethyl)formamide and N-(1-alkoxyethyl)formamide which are intermediates for N-vinylformamide. Separation of products is performed by crystallization from solution and filtration.
U.S. Pat. No. 4,578,515, Dawson et al. (1986), describes preparing ethylidene bisformamide from acetaldehyde and formamide in the presence of an acidic catalyst and an ammonia scavenger. Bisformamide is recovered by distillation and can be cracked to make N-vinylformamide, a monomer for useful polymers, such as poly(vinylamines). Thin film evaporation techniques are used because of the sensitivity of ethylidene bisformamide to high temperatures. The acid catalyst can be an acidic ion exchange resin such as sulfonated polystyrene cross-linked with divinylbenzene.
Australian Patent Application Number 68280/87, Kroenar et al. (1987), describes purifying N-vinylformamide by fractional distillation in the presence of formamide under reduced pressure so that formamide is present in the distillate. The presence of formamide during the distillation is said to avoid formation of popcorn polymer. This process requires expensive apparatus, low pressure operation and the recycle of large amounts of formamide.
Japanese Patent Application Number 62-195352, Sato et al. (1987), describes recovery of N-vinylformamide from a mixture formed on thermal decomposition of N-(.alpha.-alkoxyethyl)formamide by distillation in which acid is added to adjust the pH. This process also requires operation at low pressure and short residence times.
Japanese Patent Application Number 62-59248, Tamaru et a. (1987), discloses the production of N-vinylformamide by reacting acetaldehyde and formamide to form N-(.alpha.-hydroxyethyl)formamide which is then reacted with a polyhydric alcohol followed by thermal decomposition. The product is purified by distillation. This process requires recycle of large amounts of diol and the efficiency of the final separation of diol and N-vinylformamide has not been adequately demonstrated.
As indicated by the reference cited above, although the problems of separating amides one form another have existed for several years, there has been no suggestion or indication that amides could be separated effectively by using molecular sieves.
Tahoun, et al., Soil Science 102(4), pp. 248-54 and 102(5), pp. 314-21 (1966), discuss detection of complexes of primary, secondary and tertiary amides bonded on clay surfaces but do not suggest using the clay (montmorillonite) to separate one amide from another.
Barrer, R. M., Zeolites and Clay Minerals as Sorbents and Molecular Sieves, pp. 5-14, Academic Press (1978), gives a survey of the sorption potential of various zeolites, including types A, X, Y and ZK-5, as well as natural zeolites, such as chabazite. The presence of cations, such as Na and Ca, is said to change the molecular sieving properties of a zeolite. Separations described for chabazite include methanol from acetone; methanol, carbon disulfide and acetonitrile from benzene; ethanol from toluene; ethanol and water from diethyl ether; sulfur dioxide from chloroform; hydrogen sulfide from benzene; ethanol and methylamine form trimethylamine; ethylamine from diethylamine; acetonitrile from thiophene, hydrogen chloride form chloroform; methylene chloride from dioxane; and methylene bromide from benzene. Calcium chabazite is said to have the ability to separate n-paraffins from branched chain paraffins, cycloparaffins, and aromatic hydrocarbons, but there is no suggestion to use zeolites to separate amides.
U.S. Pat. No. 4,139,572, Miwa et al. (1979), describes separating para-xylene from other C.sub.8 aromatic hydrocarbons using a crystalline aluminosilicate adsorbent, e.g., X or Y zeolites and desorbing the para-xylene with a l-(lower alkyl)-4-isopropylbenzene.
USSR Certificate of Invention No. 724,500, Akhmadeev et al. (1980), describes a process for removing amides from dimethylformamide using, as a sorbent, activated carbon modified with boric acid. This method is said to be an improvement over purification of dimethylformamide by adsorption of impurities on silica gel or ion exchange resins followed by distillation in the presence of a non-volatile acid, water and an extractant, in which the degree of purification of product is rather low.
Breck, "Adsorption by Dehydrated Zeolite Crystals", Zeolite Molecular Sieves, pp 633-645 and 699-709, Kruger Publishing Co., Malabar, FL (1984), provides a thorough discussion of the molecular sieve effect of zeolite crystals, including the effect of various cations on adsorption performance. Also described is the use of various zeolites commerically for separation of mixtures, particularly the use of types A and X, mordenite, chabazite and erionite. The separations include drying, desulfurization, dewaxing, normal paraffin separation for detergent manufacture, hydrogen and hydrogen sulfide recovery, olefin recovery and air separation. Drying dimethylformamide is listed but there is no disclosure of separating one amide form another.
U.S. Pat. No. 4,633,018, Zinnen (1986), discloses use of a Y type zeolite cation exchanged with Ca or Ni, an X type zeolite cation exchanged with Ni, Ca, Ba, K or Na, or an L type zeolite exchanged with K for separating 2,4-toluenediamine from 2,6-toluenediamine. Reference is made to a co-pending application disclosing separation of 2,3- and 3,4-dinitrotoluene from 2,4- and 2,6-dinitrotoluene using a Ca or Na exchanged Y type zeolite. Another referenced application mentions separating 2,4-toluenediisocyanate and 2,6-toluenediisocyanate with a Y zeolite.
U.S. Pat. No. 4,575,434, Frank et al. (1986), describes a method of purifying nitriles by removing tallow amides (average molecular weight of 270) with layered aluminosilicates having exchangeable alkaline or alkaline earth cations after protonating the amides with an acid.
U.S. Pat. No. 4,714,783, Zinnen, et al. (1987), discloses separating isomers of nitrobenzaldehydes by selective adsorption on X or Y zeolites. An X-type zeolite containing sodium or lithium cations selectively adsorbs meta-nitrobenzaldehyde in preference to the para- and ortho-isomers. A Y-type zeolite having alkali metal or alkaline earth metal cations selectively adsorbs the ortho-nitrobenzaldehyde. Methyl acetate and ethyl acetate can be used as desorbents.
Ruthven, "Zeolites as Selective Adsorbents", Chem. Engr. Progress, pp. 42-50, Feb. 1988, describes the use of zeolites as adsorbents in making separations between hydrocarbons, carbohydrates and oxygen and nitrogen. The role of adsorbent pore size in selective separations is discussed and general procedures are given for adsorption processes and adsorbent regeneration. There is no disclosure on separation of amides.